1. Field of the Invention
The present invention relates to saturated cross-linked compositions and their preparation and, particularly to powderous binders for paints and coatings containing such compositions.
2. Description of the Prior Art
In the painting industry, powderous compositions are conventionally used, wherein the binder may consist of a polyester resin or an epoxide resin usually in combination with a cross-linking agent. If the polyester resin is used in combination with a cross-linking agent, wherein the functional groups are acid anhydride- and/or acid groups or masked isocyanate groups, or a cross-linking agent which is a compound derived from hexamethylol melamine, the number of hydroxyl groups predominates over the number of carboxyl groups within the polyester. If the cross-linking polyester is used without a cross-linking agent, it polycondenses with itself during the final utilization, and consequently the contents in hydroxyl groups and in carboxyl groups are more or less equal. If in the composition, the binder compprises a cross-linkable epoxide resin, the latter is combined with a cross-linking agent wherein the functional groups are acid anhydride- and/or acid groups.
The powderous painting compositions usually contain a certain amount of pigments and specific adjuvants such as spreading agents, catalysts, and coloring agents. The ratio between the amount of pigment and the amount of binder varies within a large range. The preparation of the compositions usually comprises separately preparing the binder and the cross-linking agent, optionally mixing the same at a temperature which is lower than the temperature at which the two components may react with each other, mixing the compounds of which the binder is comprised with the adjuvants by melting at a relatively low temperature, grinding the resulting mixture, and selecting a fraction of a certain particle size.
Then the final composition is distributed in powderous form onto a metal article, usually by means of an electrical pistol, then the coated article is heated in an oven to a temperature which favors the melting and spreading of the composition and the subsequent cross-linking of the binder. In this way, a smooth and glossy coating which exhibits favorable physical properties may be obtained. Yet, more and more, the painting industry requires compositions which, after baking, are able to provide a textured, non-glossy, mat or satin surface. A conventional process for achieving this purpose usually comprises introducing at least one mineral component, the particle distribution of which is chosen depending on the desired effect, into the composition in addition to the pigment. This process has important disadvantages in several aspects, namely: a high content of materials which are opposite to a good spreading of the binder, and at the upper limit may cause a lack of cohesion of the film or paint, poor mechanical properties, considerable defects with regard to adherence, and a poor resistance against solvents or aging.
The German Patent Application No. 2,351,176 discloses a composition for powderous coatings which comprises a saturated thermoplastic polyester resin which includes particles of cellulose esters which are intended to provide a mat and textured surface upon melting. Yet, this composition is not cross-linked. It necessitates a high baking temperature and its cellulose ester content is very high, which renders the resulting coating sensitive to the usual solvents. Therefore, the industry is in need of compositions which can be used as powderous binders in paints, and provide the cross-linked coating having a textured, non-glossy surface which does not exhibit the defects and insufficiencies of the heretoknown compositions.